It has long been a goal of color television designers to implement a system that automatically corrects errors in improperly modulated chrominance signals without introducing distortion during the reception of properly modulated signals. Empirical investigation has indicated than an Automatic Color Level System that is activated in response to chrominance signals having an amplitude corresponding to approximately 50% or more of maximum modulation level yields highly satisfactory results. When signal amplitudes reach a threshold level corresponding to the nominal peak-to-peak amplitude at 50% modulation, a peak detector develops a control voltage at the DC controlled amplifier in order to maintain the signal amplitude at that level. The controlled amplifier is almost universally followed by additional gain stages in the chrominance channels.
Although such an ACL system is capable of appreciable performance improvements, conventional embodiments are subject to the drawback described in the following simplified illustration:
Suppose the gain of the controlled amplifier varies from 0 to 10 decibels (dB) as the control voltage varies from 0 to 10 volts. Suppose, further, that the nominal output voltage is 1 volt and that the threshold is fixed at 2 volts. Let the normal gain of the amplifier stages following the controlled amplifier be such that the required gain of the controlled amplifier is 5 dB. The control voltage is then set for 5 volts and there is 5 dB gain variation available in either direction.
Now suppose that in a particular receiver, the gain of the chrominance amplifiers following the controlled amplifier is 3 dB lower than normal. In order to compensate, the gain of the controlled amplifier can be raised to 8 dB and its nominal output will now be 1.41 volts. Note that its output may only increase 42% (to 2 volts) before the threshold is reached and gain reduction occurs. On the other hand, if the gain following the controlled amplifier is 3 dB higher than normal, the nominal gain of the controlled amplifier will be reduced 3 dB and the normal output voltage will be 0.71 volts. The amplifier output may increase nearly 180% before the threshold is reached and gain reduction occurs. In the former situation, chrominance signals with the desired normal range of saturation will be subject to gain reduction. In the latter, over-saturated chrominance signals will not be gain reduced.